Secondary battery positive electrode active material and method for producing same

ABSTRACT

The present invention provides a positive electrode active substance for a secondary cell, the positive electrode active substance capable of suppressing adsorption of water effectively in order to obtain a high-performance lithium ion secondary cell or sodium ion secondary cell. The present invention also provides a method for producing the positive electrode active substance for a secondary cell. That is, the present invention is a positive electrode active substance for a secondary cell, in which a water-insoluble electrically conductive carbon material and carbon obtained by carbonizing a water-soluble carbon material are supported on an oxide containing at least iron or manganese, the oxide represented by formula (A) LiFe a Mn b M c PO 4 , formula (B) Li 2 Fe d Mn e N f SiO 4 , or formula (C) NaFe g Mn h Q i PO 4 .

FIELD OF THE INVENTION

The present invention relates to a positive electrode active substancefor a secondary cell, wherein both a water-insoluble electricallyconductive carbon material and carbon obtained by carbonizing awater-soluble carbon material are supported on an oxide, and a methodfor producing the positive electrode active substance for a secondarycell.

BACKGROUND OF THE INVENTION

Development of secondary cells for use in portable electronic devices,hybrid vehicles, electric vehicles, or the like is conducted, andlithium ion secondary cells in particular are widely known as the mostexcellent secondary cell which operates at around room temperature. Insuch circumstances, lithium-containing olivine type metal phosphatessuch as Li(Fe, Mn)PO₄ and Li₂(Fe, Mn)SiO₄ are not greatly affected byresource restriction and exhibit higher safety when compared withlithium transition metal oxides such as LiCoO₂, and therefore becomeoptimal positive electrode materials for obtaining high-output andlarge-capacity lithium ion secondary cells. These compounds, however,have a characteristic that it is difficult to enhance electricalconductivity sufficiently due to their crystal structures, and moreover,there is room for improvement in diffusibility of lithium ions, so thatvarious kinds of development have been conducted conventionally.

Further, in lithium ion secondary cells the spread of which isprogressing, a phenomenon is known that when the cells are left to standfor long hours after charge, the internal resistance gradually increasesto cause deterioration in cell performance. The phenomenon occursbecause water contained in cell materials at the time of production isdesorbed from the materials during repetition of charge and discharge ofthe cell, and hydrogen fluoride is produced through the chemicalreaction between the desorbed water and nonaqueous electrolytic solutionLiPF₆ with which the cell is impregnated. To suppress the deteriorationin cell performance effectively, it is also known that it is effectiveto reduce the water content in a positive electrode active substance foruse in a secondary cell (see Patent Literature 1).

Under the circumstance, for example, Patent Literature 2 discloses atechnique for reducing the water content to a predetermined value orless by conducting pulverization treatment or classification treatmentunder a dry atmosphere after pyrolysis treatment of a raw materialmixture comprising a precursor of a carbonaceous substance. Further,Patent Literature 3 discloses a technique for obtaining a compositeoxide in which an electrically conductive carbon material isprecipitated on the surface of the composite oxide uniformly byconducting mechanochemical treatment after mixing a predeterminedlithium phosphate compound, lithium silicate compound, or the like withan electrically conductive carbon material using a wet ball mill.

On the other hand, lithium is a rare and valuable substance, andtherefore various studies on sodium ion secondary cells using sodium inplace of lithium ion secondary cells have started.

For example, Patent Literature 4 discloses an active substance for asodium secondary cell using malysite type NaMnPO₄, Patent Literature 5discloses a positive electrode active substance comprising a sodiumtransition metal phosphate having an olivine type structure, and boththe literatures show that a high-performance sodium ion secondary cellis obtained.

CITATION LIST Patent Literature

[Patent Literature 11] JP-A-2013-152911

[Patent Literature 2] JP-A-2003-292309

[Patent Literature 3] JP-A-2010-218884

[Patent Literature 4] JP-A-2008-260666

[Patent Literature 5] JP-A-2011-34963

SUMMARY OF THE INVENTION Technical Problem

However, in any of the techniques described in the literatures, it foundthat because the surface of a positive electrode active substance for asecondary cell is not still completely covered with a carbon source anda portion of the surface is exposed, the adsorption of water cannot besuppressed and the water content is increased, and that it is difficultto obtain a positive electrode active substance for a secondary cellhaving a sufficiently high level of cell physical properties, such ascycle properties.

Accordingly, the subject matter of the present invention is to provide:a positive electrode active substance for a secondary cell, which cansuppress the adsorption of water effectively in order to obtain ahigh-performance lithium ion secondary cell or sodium ion secondarycell; and a method for producing the positive electrode active substancefor a secondary cell.

Solution to Problem

Thus, the present inventors have conducted various studies to find thata positive electrode active substance for a secondary cell, wherein anelectrically conductive carbon powder and carbon obtained by carbonizinga water-soluble carbon material are supported on a particular oxide, cansuppress the adsorption of water effectively because carbon derived froma plurality of carbon sources covers the surface of the oxideefficiently and therefore is extremely useful as a positive electrodeactive substance for a secondary cell, in which lithium ions or sodiumions can contribute to electrical conduction effectively. As a resultthat, they have completed the present invention.

That is, the present invention provides a positive electrode activesubstance for a secondary cell, wherein

a water-insoluble electrically conductive carbon material and carbonobtained by carbonizing a water-soluble carbon material are supported onan oxide containing at least iron or manganese, the oxide represented byformula (A), (B), or (C):

LiFe_(a)Mn_(b)M_(c)PO₄  (A)

wherein M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, orGd, and a, b, and c each represent a number satisfying 0≦a≦1, 0≦b≦1,0≦c≦0.2, 2a+2b+(valence of M)×c=2, and a+b≠0;

Li₂Fe_(d)Mn_(e)N_(f)SiO₄  (B)

wherein N represents Ni, Co, Al, Zn, V, or Zr, and d, e, and f eachrepresent a number satisfying 0≦d≦1, 0≦e≦1, 0≦f≦1, 2d+2e+(valence ofN)×f=2, and d+e≠0; and

NaFe_(g)Mn_(h)Q_(i)PO₄  (C)

wherein Q represents Mg, Ca, Co, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd,or Gd, and g, h, and i each represent a number satisfying 0≦g≦1, 0≦h≦1,0≦i<1, 2g+2h+(valence of Q)×i=2, and g+h≠0.

Moreover, the present invention provides a method for producing apositive electrode active substance for a secondary cell, the positiveelectrode active substance being one wherein a water-insolubleelectrically conductive carbon material and carbon obtained bycarbonizing a water-soluble carbon material are supported on an oxidecontaining at least iron or manganese, the oxide represented by formula(A), (B), or (C):

LiFe_(a)Mn_(b)M_(c)PO₄  (A)

wherein M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, orGd, and a, b, and c each represent a number satisfying 0≦a≦1, 0≦b≦1,0≦c≦0.2, 2a+2b+(valence of M)×c=2, and a+b≠0;

Li₂Fe_(d)Mn_(e)N_(f)SiO₄  (B)

wherein N represents Ni, Co, Al, Zn, V, or Zr, and d, e, and f eachrepresent a number satisfying 0≦d≦1, 0≦e≦1, 0≦f<1, 2d+2e+(valence ofN)×f=2, and d+e≠0; and

NaFe_(g)Mn_(h)Q_(i)PO₄  (C)

wherein Q represents Mg, Ca, Co, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd,or Gd, and g, h, and i each represent a number satisfying 0≦g≦1, 0≦h≦1,0≦i<1, 2g+2h+(valence of Q)×i=2, and g+h≠0, and

the method comprising:

a step (I) of subjecting slurry comprising: a lithium compound or asodium compound; a phosphoric acid compound or a silicic acid compound;and a metal salt comprising at least an iron compound or a manganesecompound to hydrothermal reaction, thereby obtaining an oxide X;

a step (II) of adding a water-insoluble electrically conductive carbonmaterial to the obtained oxide X and conducting dry mixing, therebyobtaining a composite Y; and

a step (III) of adding a water-soluble carbon material to the obtainedcomposite Y before wet mixing and then conducting pyrolyzing.

Effects of the Invention

According to the present invention, when a water-insoluble electricallyconductive carbon material and carbon obtained by carbonizing awater-soluble carbon material are effectively supported complementingeach other on a predetermined oxide, the exposure of the oxide in aportion of the surface of the oxide due to the absence of carbon issuppressed effectively, so that a positive electrode active substancefor a secondary cell, wherein the exposed portion on the surface of theoxide is reduced effectively, can be obtained. Therefore, the positiveelectrode active substance can suppress the adsorption of watereffectively, so that, in a lithium ion secondary cell or a sodium ionsecondary cell using the positive electrode active substance, excellentcell properties, such as cycle properties can be exhibited stably evenunder various use environments, while effectively contributing lithiumions or sodium ions to electrical conduction.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in detail.

The oxide for use in the present invention comprises at least iron ormanganese and is represented by any of the following formulas (A), (B),and (C):

LiFe_(a)Mn_(b)M_(c)PO₄  (A)

wherein M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, orGd, and a, b, and c each represent a number satisfying 0≦a≦1, 0≦b≦1,0≦c≦0.2, 2a+2b+(valence of M)×c=2, and a+b≠0;

Li₂Fe_(d)Mn_(e)N_(f)SiO₄  (B)

wherein N represents Ni, Co, Al, Zn, V, or Zr, and d, e, and f eachrepresent a number satisfying 0≦d≦1, 0≦e≦1, 0≦f<1, 2d+2e+(valence ofN)×f=2, and d+e≠0; and

NaFe_(g)Mn_(h)Q_(i)PO₄  (C)

wherein Q represents Mg, Ca, Co, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd,or Gd, and g, h, and i each represent a number satisfying 0≦g≦1, 0≦h≦1,0≦i<1, 2g+2h+(valence of Q)×i=2, and g+h≠0.

All these oxides have an olivine type structure and comprise at leastiron or manganese. In the case where the oxide represented by theformula (A) or the formula (B) is used, the positive electrode activesubstance for a lithium ion cell is obtained, and in the case where theoxide represented by the formula (C) is used, the positive electrodeactive substance for a sodium ion cell is obtained.

The oxide represented by the formula (A) is a so-called olivine typelithium transition metal phosphate compound which comprises at leastiron (Fe) and manganese (Mn) as transition metals. In the formula (A), Mrepresents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd and ispreferably Mg, Zr, Mo, or Co. a satisfies 0≦a≦1, preferably 0.01≦a≦0.99,and more preferably 0.1≦a≦0.9. b satisfies 0≦b≦1, preferably0.01≦b≦0.99, and more preferably 0.1≦b≦0.9. c satisfies 0≦c≦0.2,preferably 0≦c≦0.1. Moreover, a, b, and c are each a number satisfying2a+2b+(valence of M)×c=2 and a+b≠0. Specific examples of the olivinetype lithium transition metal phosphate compound represented by theformula (A) include LiFe_(0.2)Mn_(0.8)PO₄, LiFe_(0.9)Mn_(0.1)PO₄,LiFe_(0.15)Mn_(0.75)Mg_(0.1)PO₄, and LiFe_(0.19)Mn_(0.75)Zr_(0.03)PO₄,and among the olivine type lithium transition metal phosphate compounds,LiFe_(0.2)Mn_(0.8)PO₄ is preferable.

The oxide represented by the formula (B) is a so-called olivine typelithium transition metal silicate compound comprising at least iron (Fe)and manganese (Mn) as transition metals. In the formula (B), Nrepresents Ni, Co, Al, Zn, V, or Zr and is preferably Co, Al, Zn, V, orZr. d satisfies 0≦d≦1, preferably 0≦d<1, and more preferably 0.1≦d≦0.6.e satisfies 0≦e<1, preferably 0≦e<1, and more preferably 0.1≦e<0.6. fsatisfies 0≦f<1, preferably 0≦f≦1, and more preferably 0.05≦f≦0.4.Moreover, d, e, and f are each a number satisfying 2d+2e+(valence ofN)×f=2, and d+e≠0. Specific examples of the olivine type lithiumtransition metal silicate compound represented by the formula (B)include Li₂Fe_(0.45)Mn_(0.45)Co_(0.1)SiO₄,Li₂Fe_(0.36)Mn_(0.54)Al_(0.066)SiO₄, Li₂Fe_(0.45)Mn_(0.45)Zn_(0.1)SiO₄,Li₂Fe_(0.36)Mn_(0.54)V_(0.066)SiO₄, andLi₂Fe_(0.282)Mn_(0.585)Zr_(0.02)SiO₄, and among the olivine type lithiumtransition metal silicate compounds,Li₂Fe_(0.282)Mn_(0.658)Zr_(0.02)SiO₄ is preferable.

The oxide represented by the formula (C) is a so-called olivine typesodium transition metal phosphate compound comprising at least iron (Fe)and manganese (Mn) as transition metals. In the formula (C), Qrepresents Mg, Ca, Co, Sr, Y; Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd andis preferably Mg, Zr, Mo, or Co. g satisfies 0≦g≦1, preferably 0≦g≦1. hsatisfies 0≦h≦1, preferably 0.5≦h<1. i satisfies 0≦i<1, preferably0≦i<0.5, and more preferably 0≦i≦0.3. Moreover, g, h, and i are each anumber satisfying 0≦g≦1, 0≦h≦1, 0≦i<1, 2g+2h+(valence of Q)×i=2, andg+h≠0. Specific examples of the olivine type sodium transition metalphosphate compound represented by the formula (C) includeNaFe_(0.2)Mn_(0.8)PO₄, NaFe_(0.9)Mn_(0.1)PO₄,NaFe_(0.15)Mn_(0.7)Mg_(0.15)PO₄, NaFe_(0.19)Mn_(0.75)Zr_(0.03)PO₄,NaFe_(0.19)Mn_(0.75)Mo_(0.03)PO₄, and NaFe_(0.15)Mn_(0.7)Co_(0.15)PO₄,and among the olivine type sodium transition metal phosphate compounds,NaFe_(0.2)Mn_(0.8)PO₄ is preferable.

In the positive electrode active substance for a secondary cellaccording to the present invention, a water-insoluble electricallyconductive carbon material and carbon obtained by carbonizing awater-soluble carbon material (carbon derived from water-soluble carbonmaterial) are supported on the oxide represented by the formula (A),(B), or (C). That is, the water-insoluble electrically conductive carbonmaterial and the water-soluble carbon material coexist as carbonsources, so that carbon derived from the one carbon source covers thesurface of the oxide, and at a site where the carbon does not exist andthe surface of the oxide is exposed, carbon derived from the othercarbon source is supported effectively. Accordingly, the water-insolubleelectrically conductive carbon material together with the carbonobtained by carbonizing the water-soluble carbon material is supportedfirmly over the entire surface of the oxide while suppressing theexposure of the surface of the oxide effectively, and therefore theadsorption of water in the positive electrode active substance for asecondary cell according to the present invention can be preventedeffectively.

The water-insoluble electrically conductive carbon material to besupported on the oxide represented by the formula (A), (B), or (C) is awater-insoluble carbon material the solubility of which to 100 g ofwater at 25° C. is less than 0.4 g expressed in terms of carbon atoms ofthe water-insoluble electrically conductive carbon material and is acarbon source which itself has electrical conductivity without beingsubjected to pyrolyzing or the like. Examples of the water-insolubleelectrically conductive carbon material include at least one selectedfrom the group consisting of graphite, acetylene black, Ketjen black,channel black, furnace black, lamp black, and thermal black. Among thewater-insoluble electrically conductive carbon materials, graphite ispreferable from the viewpoint of reducing the amount of adsorbed water.The graphite may be any of artificial graphite (flake, vein, earthy,graphene) and natural graphite.

The BET specific surface area of the water-insoluble electricallyconductive carbon material which can be used is preferably 1 to 750m²/g, more preferably 3 to 500 m²/g from the viewpoint of reducing theamount of the adsorbed water effectively. In addition, the averageparticle diameter of the water-insoluble electrically conductive carbonmaterial is preferably 0.5 to 20 μm, more preferably 1.0 to 15 μm fromthe same viewpoint.

The water-soluble carbon material to be supported as carbon obtainedthrough carbonization together with the water-insoluble electricallyconductive carbon material on the oxide represented by formula (A), (B),or (C) means a carbon material which dissolves in 100 g of water at 25°C. 0.4 g or more, preferably 1.0 g or more expressed in terms of carbonatoms of the water-soluble carbon material and functions as a carbonsource which covers the surface of the oxide represented by the formulas(A) to (C). Examples of the water-soluble carbon material include atleast one selected from the group consisting of saccharides, polyols,polyethers, and organic acids. More specific examples include:monosaccharides such as glucose, fructose, galactose, and mannose;disaccharides such as maltose, sucrose, and cellobiose; polysaccharidessuch as starches and dextrins; polyols and polyethers such as ethyleneglycol, propylene glycol, diethylene glycol, polyethylene glycol, butanediol, propane diol, polyvinyl alcohol, and glycerin; and organic acidssuch as citric acid, tartaric acid, and ascorbic acid. Among thewater-soluble carbon materials, glucose, fructose, sucrose, and dextrinsare preferable, more preferably glucose from the viewpoint of highsolubility and dispersibility to solvents for functioning as a carbonmaterial effectively.

With respect to the water-insoluble electrically conductive carbonmaterial and the water-soluble carbon material each in an amountexpressed in terms of carbon atoms, the water-insoluble electricallyconductive carbon material and the carbonized water-soluble carbonmaterial coexist as the carbon supported on the oxide in the positiveelectrode active substance for a secondary cell according to the presentinvention. The amount of the water-insoluble electrically conductivecarbon material and the water-soluble carbon material expressed in termsof carbon atoms corresponds to the total amount of the supported carbonof the water-insoluble electrically conductive carbon material and thecarbon obtained by carbonizing the water-soluble carbon material and ispreferably 1.0 to 20.0 mass %, more preferably 2.0 to 17.5 mass %, andstill more preferably 3.0 to 15.0 mass % in total in the positiveelectrode active substance for a secondary cell according to the presentinvention. Specifically, the amount of the water-insoluble electricallyconductive carbon material and the water-soluble carbon materialexpressed in terms of carbon atoms is preferably 1.0 to 15.0 mass %,more preferably 2.0 to 13.5 mass %, and still more preferably 3.0 to12.0 mass % in the positive electrode active substance for a secondarycell, wherein the oxide is represented by the formula (A) or (C), andpreferably 2.0 to 20.0 mass %, more preferably 3.0 to 17.5 mass %, andstill more preferably 4.0 to 15.0 mass % in the positive electrodeactive substance for a secondary cell, wherein the oxide is representedby the formula (B).

In addition, the amount of the water-soluble carbon material expressedin terms of carbon atoms, namely the amount of carbon supported for thecarbon obtained by carbonizing the water-soluble carbon material, ispreferably 0.5 to 17.0 mass %, more preferably 0.5 to 13.5 mass %, andstill more preferably 0.5 to 10.0 mass % in the positive electrodeactive substance for a secondary cell according to the presentinvention. Specifically, the amount of the water-soluble carbon materialexpressed in terms of carbon atoms is preferably 0.5 to 10.0 mass %,more preferably 0.5 to 9.0 mass %, and still more preferably 0.5 to 8.0mass % in the positive electrode active substance for a secondary cell,wherein the oxide is represented by the formula (A) or (C), and ispreferably 0.75 to 17.0 mass %, more preferably 0.75 to 13.5 mass %, andstill more preferably 0.75 to 10.0 mass % in the positive electrodeactive substance for a secondary cell, wherein the oxide is representedby the formula (B).

It is to be noted that the total amount of the water-insolubleelectrically conductive carbon material and the water-soluble carbonmaterial which exist in the positive electrode active substance for asecondary cell expressed in terms of carbon atoms can be checked as thetotal amount of carbon measured using a carbon-sulfur analyzingapparatus. In addition, the amount of the water-soluble carbon materialexpressed in terms of carbon atoms can be checked by subtracting theamount of the water-insoluble electrically conductive carbon materialadded from the total amount of the carbon measured using a carbon-sulfuranalyzing apparatus.

In the positive electrode active substance for a secondary cellaccording to the present invention, the water-soluble carbon material ispreferably supported as carbon obtained through carbonization on thecomposite comprising the oxide represented by the formula (A), (B), or(C) and the water-insoluble electrically conductive carbon material fromthe viewpoint of allowing the water-insoluble electrically conductivecarbon material and the carbon obtained by carbonizing the water-solublecarbon material to be supported complementing each other on the oxideeffectively, and specifically, the carbon obtained by carbonizing thewater-soluble carbon material is preferably supported on the compositein which the water-insoluble electrically conductive carbon material issupported on the oxide.

Specifically, the water-insoluble electrically conductive carbonmaterial is preferably supported on the oxide by subjecting thewater-insoluble electrically conductive carbon material and the oxideobtained though hydrothermal reaction to dry mixing and is morepreferably supported on the oxide by subjecting the water-insolubleelectrically conductive carbon material and the oxide to preliminarymixing and then mixing while compressive force and shear force areapplied. That is, the composite comprising the oxide and thewater-insoluble electrically conductive carbon material is preferably amixture obtained by dry-mixing the water-insoluble electricallyconductive carbon material and the oxide which is a hydrothermalreaction product. It is to be noted that the oxide on which thewater-insoluble electrically conductive carbon material is supported bypyrolyzing the water-insoluble electrically conductive carbon materialis obtained as the composite comprising the oxide and thewater-insoluble electrically conductive carbon material. In addition,when the dry mixing is conducted, another water-soluble carbon materialmay be added supplementarily as necessary separately from thewater-soluble carbon material which is added when the wet mixing isconducted, which will be mentioned later, is conducted. The compositeobtained in this case comprises the water-soluble carbon materialtogether with the oxide and the water-insoluble electrically conductivecarbon material.

The water-soluble carbon material to be supported as the carbon obtainedthrough carbonization on the composite comprising the oxide and thewater-insoluble electrically conductive carbon material is preferablysupported on the oxide as the carbon obtained through carbonization bysubjecting the water-soluble carbon material and the composite to wetmixing and then pyrolyzing from the viewpoint of allowing carbon to besupported further effectively at the site where the water-insolubleelectrically conductive carbon material does not exist and the surfaceof the oxide is exposed in the composite. That is, the positiveelectrode active substance for a secondary cell according to the presentinvention is preferably a pyrolyzed product of a mixture obtained bywet-mixing the water-soluble carbon material with the compositecomprising the oxide and the water-insoluble electrically conductivecarbon material. By the pyrolysis for carbonizing the water-solublecarbon material, the crystallinity of both the oxide and thewater-insoluble electrically conductive carbon material which has beenlowered due to the dry mixing or the like can be recovered, andtherefore the electrical conductivity in the positive electrode activesubstance to be obtained can be enhanced effectively. It is to be notedthat the kind of water-soluble carbon material which is used when thewet mixing is conducted may be the same as or different from the kind ofwater-soluble carbon material which is used supplementarily as necessarywhen the dry mixing is conducted.

More specifically, the positive electrode active substance for asecondary cell according to the present invention is preferably obtainedby a production method comprising:

a step (I) of subjecting slurry comprising: the lithium compound or thesodium compound; a phosphoric acid compound or a silicic acid compound;and a metal salt comprising at least an iron compound or a manganesecompound to hydrothermal reaction, thereby obtaining the oxide X;

a step (II) of adding the water-insoluble electrically conductive carbonmaterial to the obtained oxide (X) and conducting dry mixing, therebyobtaining the composite Y; and

a step (III) of adding the water-soluble carbon material to the obtainedcomposite Y and conducting wet mixing and then pyrolyzing.

The step (I) is a step of subjecting the slurry comprising: the lithiumcompound or the sodium compound; the phosphoric acid compound or thesilicic acid compound; and the metal salt comprising at least the ironcompound or the manganese compound to hydrothermal reaction, therebyobtaining the oxide X.

Examples of the lithium compound or the sodium compound which can beused include hydroxides (for example, LiOH.H₂O, NaOH), carbonatedproducts, sulfonated products, and acetylated products. Among them,hydroxides are preferable.

The content of the lithium compound or the silicic acid compound in theslurry is preferably 5 to 50 mass parts, more preferably 7 to 45 massparts based on 100 mass parts of water. More specifically, in the casewhere the phosphoric acid compound is used in the step (I), the contentof the lithium compound or the sodium compound in the slurry ispreferably 5 to 50 mass parts, more preferably 10 to 45 mass parts basedon 100 mass parts of water. In the case where the silicic acid compoundis used, the content of the silicic acid compound in the slurry ispreferably 5 to 40 mass parts, more preferably 7 to 35 mass parts basedon 100 mass parts of water.

The step (1) preferably comprises a step (Ia) of mixing the phosphoricacid compound or the silicic acid compound with a mixture A¹ comprisingthe lithium compound or the sodium compound, thereby obtaining a mixtureA², and a step (Ib) of subjecting slurry X obtained by adding the metalsalt comprising at least the iron compound or the manganese compound tothe obtained mixture A² and mixing the resultant mixture to hydrothermalreaction, thereby obtaining the oxide X from the viewpoint of enhancingthe dispersibility of each component contained in the slurry and makingthe particle of the positive electrode active substance to be obtainedfine, thereby improving the cell physical properties.

It is preferable to stir the mixture A in advance before mixing thephosphoric acid compound or the silicic acid compound with the mixtureA¹ in the step (I) or (Ta). The time for stirring the mixture A¹ ispreferably 1 to 15 minutes, more preferably 3 to 10 minutes. Inaddition, the temperature of the mixture A¹ is preferably 20 to 90° C.,more preferably 20 to 70° C.

Examples of the phosphoric acid compound for use in the step (I) or (Ia)include orthophosphoric acid (H₃PO₄, phosphoric acid), metaphosphoricacid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid,ammonium phosphate, and ammonium hydrogenphosphate. Among the phosphoricacid compounds, it is preferable to use phosphoric acid, and it ispreferable to use phosphoric acid as an aqueous solution having aconcentration of 70 to 90 mass %. In the step (I) or (Ia), whenphosphoric acid is mixed with the mixture A¹, it is preferable to dropphosphoric acid while stirring the mixture A¹. When phosphoric acid isadded dropwise to the mixture A¹ little by little, the reactionprogresses in the mixture A¹ satisfactorily to produce a precursor ofthe oxide represented by the formulas (A) to (C) while the precursor ofthe oxide is uniformly dispersed in the slurry, and even unnecessaryaggregation of the precursor of the oxide can be suppressed effectively.

The speed of dropping phosphoric acid into the mixture A¹ is preferably15 to 50 mL/min, more preferably 20 to 45 mL/min, and still morepreferably 28 to 40 mL/min. In addition, the time for stirring themixture A¹ while dropping phosphoric acid is preferably 0.5 to 24 hours,more preferably 3 to 12 hours. Further, the speed of stirring themixture A¹ while dropping phosphoric acid is preferably 200 to 700 rpm,more preferably 250 to 600 rpm, and still more preferably 300 to 500rpm.

It is to be noted that when the mixture A¹ is stirred, it is preferableto cool the mixture A¹ to a temperature equal to or lower than theboiling point of the mixture A¹. Specifically, it is preferable to coolthe mixture A¹ to a temperature of 80° C. or lower, more preferably to atemperature of 20 to 60° C.

The silicic acid compound for use in the step (I) or (la) is notparticularly limited as long as the silicic acid compound is a silicacompound having reactivity, and examples include amorphous silica andNa₄SiO₄ (for example, Na₄SiO₄.H₂O).

It is preferable that the mixture A² after mixing the phosphoric acidcompound or the silicic acid compound comprise 2.0 to 4.0 mol of lithiumor sodium, more preferably 2.0 to 3.1 mol based on 1 mol of phosphoricacid or silicic acid, and the lithium compound or the sodium compound,and the phosphoric acid compound or the silicic acid compound may beused so that the amounts thereof may be as such. More specifically, inthe case where the phosphoric acid compound is used in the step (I), itis preferable that the mixture A² after mixing the phosphoric acidcompound comprise 2.7 to 3.3 mol of lithium or sodium, more preferably2.8 to 3.1 mol based on 1 mol of phosphoric acid, and in the case wherethe silicic acid compound is used in the step (I), it is preferable thatthe mixture A² after mixing the silicic acid compound comprise 2.0 to4.0 mol of lithium, more preferably 2.0 to 3.0 mol based on 1 mol ofsilicic acid.

By conducting a nitrogen purge to the mixture A² after mixing thephosphoric acid compound or the silicic acid compound, the reaction inthe mixture A² is completed to produce a precursor of the oxiderepresented by the formulas (A) to (C) in the mixture A². When thenitrogen purge is conducted, the reaction can be made to proceed in astate where the dissolved oxygen concentration in the mixture A² isreduced, and moreover, the dissolved oxygen concentration in the mixturewhich comprises the obtained precursor of the oxide is also reducedeffectively, so that oxidation of the iron compound, the manganesecompounds, and the like to be added in the next step can be suppressed.In the mixture A², the precursor of the oxide represented by theformulas (A) to (C) exists as a fine dispersed particle. The precursorof the oxide is obtained, for example, as a trilithium phosphate(Li₃PO₄) in the case of the oxide represented by the formula (A).

The pressure in conducting the nitrogen purge is preferably 0.1 to 0.2MPa, more preferably 0.1 to 0.15 MPa. In addition, the temperature ofthe mixture A² after mixing the phosphoric acid compound or the silicicacid compound is preferably 20 to 80° C., more preferably 20 to 60° C.For example, in the case of the oxide represented by the formula (A),the reaction time is preferably 5 to 60 minutes, more preferably 15 to45 minutes.

In addition, when the nitrogen purge is conducted, it is preferable tostir the mixture A² after mixing the phosphoric acid compound or thesilicic acid compound from the viewpoint of allowing the reaction toprogress satisfactorily. The stirring speed in this case is preferably200 to 700 rpm, more preferably 250 to 600 rpm.

In addition, it is preferable to make the dissolved oxygen concentrationin the mixture A² after mixing the phosphoric acid compound or thesilicic acid compound 0.5 mg/L or lower, more preferably 0.2 mg/L orlower from the viewpoint of suppressing the oxidation at the surface ofthe dispersed particle of the precursor of the oxide more effectivelyand making the dispersed particle fine.

In the step (I) or (Ib), the slurry comprising: the obtained precursorof the oxide; and the metal salt comprising at least the iron compoundor the manganese compound is subsequently subjected to hydrothermalreaction, thereby obtaining the oxide X. It is preferable that theobtained precursor of the oxide be used as it is as the mixture and themetal salt comprising at least the iron compound or the manganesecompound be added to the precursor of the oxide to prepare the slurry X.Thereby, the oxide represented by the formulas (A) to (C) can beobtained, and the particle of the oxide can be made extremely fine whilethe steps are simplified, so that an extremely useful positive electrodeactive substance for a secondary cell can be obtained.

Examples of the iron compound which can be used include iron acetate,iron nitrate, and iron sulfate. These iron compounds may be used singlyor in a combination of two or more. Among the iron compounds, ironsulfate is preferable from the viewpoint of enhancing cell properties.

Examples of the manganese compound which can be used include manganeseacetate, manganese nitrate, and manganese sulfate. These manganesecompounds may be used singly or in a combination of two or more. Amongthe manganese compounds, manganese sulfate is preferable from theviewpoint of enhancing the cell properties.

In the case where both the iron compound and the manganese compound areused as the metal salt, the molar ratio of the iron compound used andthe manganese compound used (iron compound:manganese compound) ispreferably 99:1 to 1:99, more preferably 90:10 to 10:90. In addition,the total amount of the iron compound and the manganese compound addedis preferably 0.99 to 1.01 mol, more preferably 0.995 to 1.005 mol basedon 1 mol of Li₃PO₄ contained in the slurry X.

Further, a metal (M, N, or Q) salt other than the iron compound and themanganese compound may be used as the metal salt as necessary. In themetal (M, N, or Q) salt, M, N, and Q have the same meaning as M, N, andQ in the formulas (A) to (C), and as the metal salt, sulfates, halogencompounds, organic acid salts, hydrates thereof, and the like can beused. These metal salts may be used singly, or two or more of thesemetal salts may be used. Among the metal salts, sulfates are morepreferably used from the viewpoint of enhancing the cell properties.

In the case where these metal (M, N, or Q) salts are used, the totalamount of the iron compound, manganese compound, and metal salts (M, N,or Q) added is preferably 0.99 to 1.01 mol, more preferably 0.995 to1.005 mol based on 1 mol of phosphoric acid or silicic acid in themixture obtained through the step (I).

The amount of water for use in conducting the hydrothermal reaction ispreferably 10 to 50 mol, more preferably 12.5 to 45 mol based on 1 molof phosphoric acid ion or silicic acid ion contained in the slurry Xfrom the viewpoint of solubility of the metal salt to be used, easinessof stirring, efficiency of synthesis, and the like. More specifically,in the case where the ion contained in the slurry X is a phosphate ion,the amount of water for use in conducting the hydrothermal reaction ispreferably 10 to 30 mol, more preferably 12.5 to 25 mol. In the casewhere the ion contained in the slurry X is a silicate ion, the amount ofwater for use in conducting the hydrothermal reaction is preferably 10to 50 mol, more preferably 12.5 to 45 mol.

In the step (I) or (Ib), the order of addition of the iron compound, themanganese compound, and the metal (M, N, or Q) salt is not particularlylimited. In addition, an antioxidant may be added as necessary withthese metal salts. As the antioxidant, sodium sulfite (Na₂SO₃), sodiumhydrosulfite (Na₂S₂O₄), ammonia water, and the like can be used. Theamount of the antioxidant added is preferably 0.01 to 1 mol, morepreferably 0.03 to 0.5 mol based on 1 mol of the total amount of theiron compound, manganese compound, and the metal (M, N, or Q) salt whichis added as necessary from the viewpoint of preventing suppression ofthe production of the oxide represented by the formulas (A) to (C)caused by excessive addition of the antioxidant.

The content of the precursor of the oxide in the slurry X obtained byadding the iron compound, the manganese compound, and the metal (M, N,or Q) salt or the antioxidant which is used as necessary is preferably10 to 50 mass %, more preferably 15 to 45 mass %, and more preferably 20to 40 mass %.

The temperature during the hydrothermal reaction in the step (I) or (Ib)may be 100° C. or higher, more preferably 130 to 18000. It is preferablethat the hydrothermal reaction be conducted in a pressure resistantcontainer. In the case where the reaction is conducted at 130 to 180°C., it is preferable that the pressure during the reaction be 0.3 to 0.9MPa, and in the case where the reaction is conducted at 140 to 160° C.,it is preferable that the pressure during the reaction be 0.3 to 0.6MPa. It is preferable that the time for the hydrothermal reaction be 0.1to 48 hours, more preferably 0.2 to 24 hours.

The obtained oxide X is the oxide represented by the formulas (A) to(C), and the oxide X can be isolated through washing with water afterfiltration, and drying thereafter. It is to be noted that as dry means,freeze dry and vacuum dry are used.

The BET specific surface area of the oxide X obtained is preferably 5 to40 m²/g, more preferably 5 to 20 m²/g from the viewpoint of allowing thecarbon which coexists with the oxide X to be supported efficiently andreducing the amount of the adsorbed water effectively. When the BETspecific surface area of the oxide X is less than 5 m²/g, there is arisk that the primary particle of the positive electrode activesubstance for a secondary cell becomes too large and the cell propertiesare lowered. When the BET specific surface area exceeds 40 m²/g, thereis a risk that the amount of the adsorbed water in the positiveelectrode active substance for a secondary cell increases to give aninfluence on the cell properties.

The step (II) is a step of adding the water-insoluble electricallyconductive carbon material to the oxide X obtained in the step (I) andthen conducting dry mixing, thereby obtaining the composite Y. Thewater-soluble carbon material may further be added supplementarily inaddition to the water-insoluble electrically conductive carbon material,and in this case, the order of addition of these materials is notparticularly limited. The amount of the water-insoluble electricallyconductive carbon material added is, for example, preferably 0.5 to 24.2mass parts, more preferably 1.5 to 20.5 mass parts, and still morepreferably 2.6 to 17.0 mass parts based on 100 mass parts of the oxideX. Specifically, the amount of the water-insoluble electricallyconductive carbon material added is preferably 0.5 to 17.0 mass parts,more preferably 1.5 to 14.9 mass parts, and still more preferably 2.6 to13.0 mass parts in the positive electrode active substance for asecondary cell, wherein the oxide is represented by the formula (A) or(C), and preferably 1.3 to 23.8 mass parts, more preferably 2.3 to 20.1mass parts, and still more preferably 3.4 to 16.6 mass parts in thepositive electrode active substance for a secondary cell, wherein theoxide is represented by the formula (B).

In addition, in the case where the water-insoluble electricallyconductive carbon material and the water-soluble carbon material areused together in the step (II), the mass ratio of the amount of thewater-insoluble electrically conductive carbon material and the amountof the water-soluble carbon material added and expressed in terms ofcarbon atoms (water-insoluble electrically conductive carbon material:water-soluble carbon material) is preferably 100:2 to 3:100, morepreferably 100:10 to 10:100.

The dry mixing in the step (II) is preferably mixing with an ordinaryball mill, more preferably mixing with a planetary ball mill capable ofrotating and revolving. Further, the composite Y is more preferablymixed while the compressive force and the shear force are applied toprepare a composite Y′ from the viewpoint of dispersing thewater-insoluble electrically conductive carbon material and thewater-soluble carbon material used together as necessary densely anduniformly on the surface of the oxide represented by the formulas (A) to(C), thereby allowing the carbon materials to be supported effectivelyas the carbon obtained through carbonization. It is preferable toconduct the mixing treatment which is conducted while the compressiveforce and the shear force are applied in an airtight container providedwith an impeller. The circumferential speed of the impeller ispreferably 25 to 40 m/s, more preferably 27 to 40 m/s from the viewpointof enhancing the tap density of the positive electrode active substanceto be obtained and reducing the BET specific surface area to reduce theamount of the adsorbed water effectively. In addition, the mixing timeis preferably 5 to 90 minutes, more preferably 10 to 80 minutes.

It is to be noted that the circumferential speed of the impeller meansthe speed of the outermost edge portion of a rotary type stirring blade(impeller) and can be expressed by the following formula (1), and thetime for conducting the mixing treatment while applying the compressiveforce and the shear force becomes longer as the circumferential speed ofthe impeller becomes slower and therefore can be varied depending on thecircumferential speed of the impeller.

Circumferential speed of impeller (m/s)=Radius of impeller(m)×2×π×number of revolution (rpm)÷60  (1)

The treatment time and/or the circumferential speed of the impeller inconducting the mixing treatment while applying the compressive force andthe shear force in the step (II) need to be adjusted appropriatelyaccording to the amount of the composite Y which is put into thecontainer. By operating the container, the treatment of mixing themixture can be conducted while the compressive force and the shear forceare applied to the mixture between the impeller and the inner wall ofthe container, so that the water-insoluble electrically conductivecarbon material and the water-soluble carbon material which is usedsupplementarily as necessary are densely and uniformly dispersed on thesurface of the oxide represented by the formulas (A) to (C), and thepositive electrode active substance for a secondary cell, in which theamount of the adsorbed water can be reduced effectively by thewater-insoluble electrically conductive carbon material together withthe water-soluble carbon material which is to be added in the step(III), as will be mentioned later, can be obtained.

For example, in the case where the mixing treatment is conducted in anairtight container provided with an impeller rotating at acircumferential speed of 25 to 40 m/s for 6 to 90 minutes, the amount ofthe composite Y put into the container is preferably 0.1 to 0.7 g, morepreferably 0.15 to 0.4 g per 1 cm³ of the effective container (in thecontainer provided with the impeller, a container corresponding to asite where the composite Y can be accommodated).

Examples of an apparatus provided with the airtight container in whichthe mixing treatment can be conducted easily while the compressive forceand the shear force are applied include a high-speed shearing mill and ablade type kneader, and specifically, for example, a particle composingmachine, Nobilta (manufactured by Hosokawa Micron Corporation), can beused suitably.

With respect to the mixing treatment conditions, the treatmenttemperature is preferably 5 to 80° C., more preferably 10 to 50° C. Thetreatment atmosphere is not particularly limited; however, the treatmentis preferably conducted under an inert gas atmosphere or a reducing gasatmosphere.

The step (III) is a step of adding the water-soluble carbon material toa product obtained in the step (I) and conducting wet mixing and thenpyrolyzing. Through the step (III), the exposure of the surface of theoxide X represented by the formulas (A) to (C) is suppressedeffectively, and both the water-insoluble electrically conductive carbonmaterial and the carbon obtained by carbonizing the water-soluble carbonmaterial can be supported on the oxide X firmly.

The amount of the water-soluble carbon material added in the step (III)is preferably 1.0 to 55.0 mass parts, more preferably 1.0 to 40.0 massparts, and still more preferably 1.0 to 30.0 mass parts based on 100mass parts of the composite Y (or composite Y′ in the case where themixing treatment which is conducted while the compressive force and theshear force are applied is further conducted in the step (III), the sameapplies hereinafter) from the viewpoint of allowing the carbon obtainedby carbonizing the water-soluble carbon material to be supportedeffectively on the surface of the oxide X where the water-insolubleelectrically conductive carbon material does not exist and keeping asufficient charge and discharge capacity.

In the step (III), it is preferable to add water with the water-solublecarbon material from the viewpoint of allowing the carbon obtained bycarbonizing the water-soluble carbon material to be supported on thesurface of the oxide further satisfactorily. The amount of water addedis preferably 30 to 300 mass parts, more preferably 50 to 250 massparts, and still more preferably 75 to 200 mass parts based on 100 massparts of the composite Y (or composite Y′). The water dissolves thewater-soluble carbon material added supplementarily and supported on thecomposite Y (or composite Y′), and allows the water-soluble carbonmaterial to exhibit the same action as the action of the water-solublecarbon material added in the step (III) in the case where thewater-soluble carbon material is added supplementarily in the step (II).

The wet mixing means in the step (III) is not particularly limited, andthe wet mixing can be conducted by an ordinary method. The temperatureduring the mixing after adding the water-soluble carbon material to thecomposite Y (or composite Y′) is preferably 5 to 80° C., more preferably7 to 70° C. It is preferable to dry the obtained mixture beforepyrolyzing. Examples of the dry means include spray dry, vacuum dry, andfreeze dry.

In the step (III), the mixture obtained through the wet mixing ispyrolyzed. It is preferable to conduct pyrolysis in a reducingatmosphere or an inert atmosphere. The pyrolysis temperature ispreferably 500 to 800° C., more preferably 600 to 770° C., and stillmore preferably 650 to 750° C. from the viewpoint of enhancing thecrystallinity of the water-insoluble electrically conductive carbonmaterial to improve the electrical conductivity and the viewpoint ofcarbonizing the water-soluble carbon material more effectively. Inaddition, the pyrolysis time is preferably 10 minutes to 3 hours, morepreferably 30 minutes to 1.5 hours.

In the positive electrode active substance for a secondary cellaccording to the present invention, both the water-insolubleelectrically conductive carbon material and the carbon obtained bycarbonizing the water-soluble carbon material are supported on the oxideand act synergistically, so that the amount of the adsorbed water in thepositive electrode active substance for a secondary cell can be reducedeffectively. Specifically, in the positive electrode active substancefor a secondary cell, wherein the oxide is represented by the formula(A) or (C), the amount of the absorbed water in the positive electrodeactive substance for a secondary cell according to the present inventionis preferably 850 ppm or less, more preferably 700 ppm or less, and inthe positive electrode active substance for a secondary cell, whereinthe oxide is represented by the formula (B), the amount is preferably2,900 ppm or less, more preferably 2,500 ppm or less. It is to be notedthat the amount of the adsorbed water is a value measured as the amountof water volatilized between a start point and an end point, whereinwhen water is adsorbed at a temperature of 20° C. and a relativehumidity of 50% until an equilibrium is achieved, the temperature isthen raised to 150° C. where the temperature is kept for 20 minutes, andthe temperature is then further raised to 2500° C. where the temperatureis kept for 20 minutes, the start point is defined as the time whenraising the temperature is restarted from 150° C., and the end point isdefined as the time when the state of the constant temperature at 250°C. is completed. The amount of the adsorbed water in the positiveelectrode active substance for a secondary cell and the amount of thewater volatilized between the start point and the end point are regardedas the same amount, and the measured value of the amount of the watervolatilized is defined as the amount of the adsorbed water in thepositive electrode active substance for a secondary cell.

As described above, the positive electrode active substance for asecondary cell according to the present invention is hard to adsorbwater, and therefore the amount of the adsorbed water can be reducedeffectively without a strong drying condition as a productionenvironment and excellent cell properties can be exhibited stably evenunder the various use environments in both the lithium ion secondarycell and the sodium ion secondary cell to be obtained.

It is to be noted that the amount of the water volatilized between thestart point and the end point, wherein when water is adsorbed at atemperature of 20° C. and a relative humidity of 50% until anequilibrium is achieved, the temperature is then raised to 150° C. wherethe temperature is kept for 20 minutes, and the temperature is thenfurther raised to 250° C. where the temperature is kept for 20 minutes,the start point is defined as the time when raising the temperature isrestarted from 150° C., and the end point is defined as the time whenthe state of the constant temperature at 250° C. is completed, can bemeasured, for example, using a Karl Fischer moisture titrator.

In addition, the tap density of the positive electrode active substancefor a secondary cell according to the present invention is preferably0.5 to 1.6 g/cm³, more preferably 0.8 to 1.6 g/cm³ from the viewpoint ofreducing the amount of the adsorbed water effectively.

Further, the BET specific surface area of the positive electrode activesubstance for a secondary cell according to the present invention ispreferably 5 to 21 m²/g, more preferably 7 to 20 m²/g from the viewpointof reducing the amount of the adsorbed water effectively.

The secondary cell to which a positive electrode for a secondary cell,the positive electrode comprising the positive electrode activesubstance for a secondary cell according to the present invention, isapplicable is not particularly limited as long as the secondary cellcomprises a positive electrode, a negative electrode, an electrolyticsolution, and a separator as essential constituents.

The negative electrode here is not particularly limited by the materialconstitution thereof as long as the negative electrode can occludelithium ions or sodium ions during charge and release lithium ions orsodium ions during discharge, and negative electrodes having publiclyknown material constitution can be used. Examples of the materialinclude lithium metal, sodium metal, and a carbon material such asgraphite or amorphous carbon. It is preferable to use an electrode, or acarbon material in particular, which is formed from an intercalationmaterial that can electrochemically occlude-release lithium ions orsodium ions.

The electrolytic solution is obtained by dissolving a supportingelectrolyte in an organic solvent. The organic solvent is notparticularly limited as long as the organic solvent is usually used inan electrolytic solution for a lithium ion secondary cell or a sodiumion secondary cell, and for example, carbonates, halogenatedhydrocarbons, ethers, ketones, nitriles, lactones, and oxolane compoundscan be used.

The kind of the supporting electrolyte is not particularly limited;however, in the case of lithium ion secondary cells, at least one ofinorganic salts selected from the group consisting of LiPF₅, LiBF₄,LiClO₄, and LiAsF₆ and derivatives of the inorganic salts; and organicsalts selected from the group consisting of LiSO₃CF₃, LiC(SO₃CF₃)₂,LiN(SO₃CF₃)₂, LiN(SO₂C₂F₅)₂, and LiN(SO₂CF₃)(SO₂C₄F₉) and derivatives ofthe organic salts is preferable. In addition, in the case of sodium ionsecondary cells, at least one of inorganic salts selected from the groupconsisting of NaPF₆, NaBF₄, NaClO₄, and NaAsF₆ and derivatives of theinorganic salts; and organic salts selected from the group consisting ofNaSO₃CF₃, NaC(SO₃CF₃)₂, NaN(SO₃CF₃)₂, NaN(SO₂C₂F₅)₂, andNaN(SO₂CF₃)(SO₂C₄F₉) and derivatives of the organic salts is preferable.

The separator plays a roll of electrically insulating the positiveelectrode and the negative electrode and holding the electrolyticsolution. For example, a porous synthetic resin membrane, a porousmembrane of a polyolefin-based polymer (polyethylene, polypropylene) inparticular, may be used.

EXAMPLES

Hereinafter, the present invention will be described specifically basedon Examples; however, the present invention is not limited to theExamples.

Example 1-1

Slurry was obtained by mixing 4.9 Kg of LiOH.H₂O and 11.7 Kg of water.Subsequently, 5.09 Kg of a 70% phosphoric acid aqueous solution wasadded dropwise to the obtained slurry at 35 mL/min while the obtainedslurry was stirred at a speed of 400 rpm for 30 minutes, during whichthe temperature was kept at 25° C., to obtain a mixture A¹. The mixedslurry solution had a pH of 10.0 and comprised 0.33 mol of phosphoricacid based on 1 mol of lithium hydroxide.

Subsequently, the obtained mixture A¹ was purged with nitrogen while theobtained mixture A¹ was stirred at a speed of 400 rpm for 30 minutes tocomplete the reaction in the mixture A¹ (dissolved oxygen concentrationof 0.5 mg/L). Subsequently, 1.63 kg of FeSO₄.7H₂O. and 5.60 kg ofMnSO₄.H₂O were added to 21.7 kg of the mixture A¹, 0.0468 kg of Na₂SO₃was further added, and the resultant mixture was stirred and mixed at aspeed of 400 rpm to obtain a mixture A². In this case, the molar ratioof FeSO₄-7H₂O added and MnSO₄.H₂O added (FeSO₄.7H₂O:MnSO₄.H₂O) was20:80.

Subsequently, the mixture A² was put into a synthesis containerinstalled in a steam heating type autoclave. After the mixture was putinto the synthesis container, the mixture was heated while stirring at170° C. for 1 hour using saturated steam obtained by heating water(dissolved oxygen concentration of less than 0.5 mg/L) with a diaphragmseparation apparatus. The pressure in the autoclave was 0.8 MPa. Aproduced crystal was filtered and then washed with water. The washedcrystal was subjected to vacuum dry under conditions of 60° C. and 1Torr to obtain an oxide X¹ (powder, chemical composition represented byformula (A): LiFe_(0.2)Mn_(0.8)PO₄).

The obtained oxide X¹ in an amount of 100 g was taken out and was thensubjected to dry mixing with 4 g of graphite (high-purity graphitepowder, manufactured by Nippon Graphite Industries, Co., Ltd., BETspecific surface area of 5 m²/g, average particle diameter of 6.1 μm,corresponding to 3.8 mass % expressed in terms of carbon atoms in activesubstance) using a ball mill. Mixing treatment was conducted to anobtained composite Y¹ using Nobilta (manufactured by Hosokawa MicronCorporation, NOB130) at 40 m/s (6,000 rpm) for 5 minutes to obtain acomposite Y¹′ (powder).

The obtained composite Y¹′ in an amount of 10 g was taken out, 0.25 g(corresponding to 1.0 mass % expressed in terms of carbon atoms inactive substance) of glucose and 10 mL of water were then added thereto,and the resultant mixture was mixed, then dried at 80° C. for 12 hours,and then pyrolyzed at 700° C. for 11 hours in a reducing atmosphere toobtain a positive electrode active substance (LiFe_(0.2)Mn_(0.8)PO₄,amount of carbon=4.8 mass %) for a lithium ion secondary cell, whereinthe graphite and carbon derived from glucose were supported on the oxideX¹.

Example 1-2

A positive electrode active substance (LiFe_(0.2)Mn_(0.8)PO₄, amount ofcarbon=5.8 mass %) for a lithium ion secondary cell was obtained in thesame manner as in Example 1-1 except that the amount of glucose added tothe composite Y²′ was changed to 0.5 g (corresponding to 2.0 mass %expressed in terms of carbon atoms in active substance).

Example 1-3

A positive electrode active substance (LiFe_(0.2)Mn_(0.8)PO₄, amount ofcarbon=6.7 mass %) for a lithium ion secondary cell was obtained in thesame manner as in Example 1-1 except that the amount of glucose added tothe composite Y¹′ was changed to 0.75 g (corresponding to 2.9 mass %expressed in terms of carbon atoms in active substance).

Example 1-4

A positive electrode active substance (LiFe_(0.2)Mn_(0.8)PO₄, amount ofcarbon=8.6 mass %) for a lithium ion secondary cell was obtained in thesame manner as in Example 1-1 except that the amount of glucose added tothe composite Y¹′ was changed to 1.25 g (corresponding to 4.8 mass %expressed in terms of carbon atoms in active substance).

Example 1-5

An oxide X² (powder, chemical composition represented by formula (A):LiFe_(0.9)Mn_(0.1)PO₄) was obtained in the same manner as in Example 1-1except that the amount of FeSO₄.7H₂O was changed to 7.34 kg, and theamount of MnSO₄—H₂O was changed to 0.7 kg, and then 4 g of the graphite(corresponding to 3.8 mass % expressed in terms of carbon atoms inactive substance) was mixed thereto to obtain a composite Y² and acomposite Y²′ in the same manner as in Example 1-1. Subsequently, 0.25 g(corresponding to 1.0 mass % expressed in terms of carbon atoms inactive substance) glucose was added thereto to obtain a positiveelectrode active substance (LiFe_(0.9)Mn_(0.1)PO₄, amount of carbon=4.8mass %) for a lithium ion secondary cell, wherein the graphite and thecarbon derived from glucose were supported.

Comparative Example 1-1

A positive electrode active substance (LiFe_(0.9)Mn_(0.1)PO₄, amount ofcarbon=3.8 mass %) for a lithium ion secondary cell was obtained in thesame manner as in Example 1-5 except that glucose was not added to thecomposite Y²′.

Example 2-11

Slurry was obtained by mixing 3.75 L of ultrapure water with 0.428 kg ofLiOH.H₂O and 1.40 kg of Na₄SiO₄.nH₂O. To the slurry, 0.39 kg ofFeSO₄.7H₂O, 0.79 kg of MnSO₄.5H₂O, and 0.053 kg of Zr(SO₄)₂.4H₂O wereadded, and the resultant mixture was mixed. Subsequently, an obtainedmixed solution was put into an autoclave to conduct hydrothermalreaction at 150° C. for 12 hours. The pressure in the autoclave was 0.4MPa. A produced crystal was filtered and then washed with 12 mass partsof water based on 1 mass part of the crystal. The washed crystal wassubjected to freeze dry at −50° C. for 12 hours to obtain an oxide X³(powder, chemical composition represented by formula (B):Li₂Fe_(0.28)Mn_(0.66)Zr_(0.03)SiO₄).

The obtained oxide X³ in an amount of 213.9 g was taken out and was thensubjected to dry mixing with 16.1 g (corresponding to 7.0 mass %expressed in terms of carbon atoms in active substance) of the graphiteusing the ball mill. Mixing treatment was conducted to an obtainedcomposite Y³ using Nobilta (manufactured by Hosokawa Micron Corporation,NOB130) at 40 m/s (6,000 rpm) for 5 minutes to obtain a composite Y³′(powder). The obtained composite Y³′ in an amount of 5 g was taken out,0.125 g (corresponding to 1.0 mass % expressed in terms of carbon atomsin active substance) of glucose and 10 ml of water were then addedthereto, and the resultant mixture was mixed, then dried at 80° C. for12 hours, and then pyrolyzed at 650° C. for 1 hour in a reducingatmosphere to obtain a positive electrode active substance(Li₂Fe_(0.28)Mn_(0.66)Zr_(0.03)SiO₄, amount of carbon=8.0 mass %) for alithium ion secondary cell, wherein the graphite and the carbon derivedfrom glucose were supported on the oxide X³.

Example 2-2

A positive electrode active substance(Li₂Fe_(0.28)Mn_(0.66)Zr_(0.03)SiO₄, amount of carbon=9.0 mass %) for alithium ion secondary cell was obtained in the same manner as in Example2-1 except that the amount of glucose added to the composite Y³′ waschanged to 0.25 g (corresponding to 2.0 mass % expressed in terms ofcarbon atoms in active substance).

Example 2-3

A positive electrode active substance(Li₂Fe_(0.28)Mn_(0.66)Zr_(0.03)SiO₄, amount of carbon=9.9 mass %) for alithium ion secondary cell was obtained in the same manner as in Example2-1 except that the amount of glucose added to the composite Y³′ waschanged to 0.375 g (corresponding to 2.9 mass % expressed in terms ofcarbon atoms in active substance).

Example 2-4

A positive electrode active substance(Li₂Fe_(0.28)Mn_(0.66)Zr_(0.03)SiO₄, amount of carbon=13.8 mass %) for alithium ion secondary cell was obtained in the same manner as in Example2-1 except that the amount of glucose added to the composite Y³′ waschanged to 0.875 g (corresponding to 6.8 mass % expressed in terms ofcarbon atoms in active substance).

Comparative Example 2-1

A positive electrode active substance(Li₂Fe_(0.28)Mn_(0.66)Zr_(0.03)SiO₄, amount of carbon=7.0 mass %) for alithium ion secondary cell was obtained in the same manner as in Example2-1 except that glucose was not added to the composite Y³′.

Example 3-1

Solution was obtained by mixing 0.60 kg of NaOH and 9.0 L of water.Subsequently, 0.577 kg of the 85% phosphoric acid aqueous solution wasadded dropwise to the obtained solution at 35 mL/min while the obtainedsolution was stirred for 5 minutes, during which the temperature waskept at 25° C., and subsequently, the resultant mixture was stirred at aspeed of 400 rpm for 12 hours to obtain slurry comprising a mixture A⁴.The slurry comprised 3.00 mol of sodium based on 1 mol of phosphorus.The nitrogen gas purge was conducted to the obtained slurry to adjustthe dissolved oxygen concentration to 0.5 mg/L, 0.139 kg of FeSO₄.7H₂O,0.964 kg of MnSO₄.5H₂O, and 0.124 kg of MgSO₄.7H₂O were then added tothe slurry. Subsequently, the obtained mixed solution was put into theautoclave purged with a nitrogen gas to conduct hydrothermal reaction at200° C. for 3 hours. The pressure in the autoclave was 1.4 MPa. Aproduced crystal was filtered and then washed with 12 mass parts ofwater based on 1 mass part of the crystal. The washed crystal wassubjected to freeze dry at −50° C. for 12 hours to obtain an oxide X⁴(powder, chemical composition represented by formula (C):NaFe_(0.1)Mn_(0.8)Mg_(0.1)PO₄).

The obtained oxide X⁴ in an amount of 153.6 g was taken out and was thensubjected to dry mixing with 6.4 g (corresponding to 4 mass % expressedin terms of carbon atoms in active substance) of the graphite using theball mill. Mixing treatment was conducted to the composite Y⁴ usingNobilta (manufactured by Hosokawa Micron Corporation, NOB130) at 40 m/s(6,000 rpm) for 5 minutes to obtain a composite Y⁴′ (powder). Theobtained composite Y⁴′ in an amount of 5 g was taken out, 0.125 g(corresponding to 1.0 mass % expressed in terms of carbon atoms inactive substance) of glucose and 10 mL of water were then added thereto,and the resultant mixture was mixed, then dried at 80° C. for 12 hours,and then pyrolyzed at 700° C. for 1 hour in a reducing atmosphere toobtain a positive electrode active substance(NaFe_(0.1)Mn_(0.8)Mg_(0.1)PO₄, amount of carbon=5.0 mass %) for asodium ion secondary cell, wherein the graphite and the carbon derivedfrom glucose were supported on the oxide X⁴.

Example 3-2

A positive electrode active substance (NaFe_(0.1)Mn_(0.8)Mg_(0.1)PO₄,amount of carbon=6.0 mass %) for a sodium ion secondary cell wasobtained in the same manner as in Example 3-1 except that the amount ofglucose added to the composite Y⁴′ was changed to 0.25 g (correspondingto 2.0 mass % expressed in terms of carbon atoms in active substance).

Example 3-3

A positive electrode active substance (NaFe_(0.1)Mn_(0.8)Mg_(0.1)PO₄,amount of carbon=6.9 mass %) for a sodium ion secondary cell wasobtained in the same manner as in Example 3-1 except that the amount ofglucose added to the composite Y⁴′ was changed to 0.375 g (correspondingto 2.9 mass % expressed in terms of carbon atoms in active substance).

Example 3-4

A positive electrode active substance (NaFe_(0.1)Mn_(0.8)Mg_(0.1)PO₄,amount of carbon=10.8 mass %) for a sodium ion secondary cell wasobtained in the same manner as in Example 3-1 except that the amount ofglucose added to the composite Y⁴′ was changed to 0.92 g (correspondingto 6.8 mass % expressed in terms of carbon atoms in active substance).

Comparative Example 3-1

A positive electrode active substance (NaFe_(0.1)Mn_(0.8)Mg_(0.1)PO₄,amount of carbon=4.0 mass %) for a sodium ion secondary cell wasobtained in the same manner as in Example 3-1 except that glucose wasnot added to the composite Y⁴′.

<<Measurement of Amount of Adsorbed Water>>

The amount of the adsorbed water for each positive electrode activesubstance obtained in Examples 1-1 to 3-4 and Comparative Examples 1-1to 3-1 was measured in accordance with the following method.

The amount of water volatilized between a start point and an end point,in which when the positive electrode active substance (compositeparticle) was left to stand in an environment of a temperature of 20° C.and a relative humidity of 50% for one day to adsorb water until anequilibrium was achieved, the temperature was then raised to 150° C.where the temperature was kept for 20 minutes, and the temperature wasthen further raised to 250° C. where the temperature was kept for 20minutes, the start point is defined as the time when raising thetemperature was restarted from 150° C., and the end point is defined asthe time when the state of the constant temperature at 250° C. wascompleted, was measured with a Karl Fischer moisture titrator (MKC-610,manufactured by Kyoto Electronics Manufacturing Co., Ltd.) to determinethe amount of the adsorbed water in the positive electrode activesubstance.

The results are shown in Table 1.

TABLE 1 Amount supported in 100 mass % of active substance (mass %)Water- Water-soluble 250° C. insoluble carbon material Amountelectrically (expressed of conductive in terms of water carbon materialcarbon atoms) (ppm) Example 1-1 3.8 1.0 410 Example 1-2 3.8 2.0 360Example 1-3 3.8 2.9 340 Example 1-4 3.8 4.8 500 Example 1-5 3.8 1.0 400Comparative 3.8 0.0 1350 Example 1-1 Example 2-1 7.0 1.0 2120 Example2-2 7.0 2.0 1580 Example 2-3 7.0 2.9 2200 Example 2-4 7.0 6.8 2810Comparative 7.0 0.0 3080 Example 2-1 Example 3-1 4.0 1.0 370 Example 3-24.0 2.0 300 Example 3-3 4.0 2.9 410 Example 3-4 4.0 6.8 810 Comparative4.0 0.0 1870 Example 3-1

<<Evaluation of Charge and Discharge Properties Using Secondary Cells>>

Positive electrodes for a lithium ion secondary cell or a sodium ionsecondary cell were prepared using each positive electrode activesubstance obtained in Examples 1-1 to 3-4 and Comparative Examples 1-1to 3-1. Specifically, the obtained positive electrode active substance,the Ketjen black, and polyvinylidene fluoride were mixed in a blendingratio of 75:20:5 in terms of a mass ratio, N-methyl-2-pyrrolidone wasthen added thereto, and the resultant mixture was kneaded sufficientlyto prepare a positive electrode slurry. The positive electrode slurrywas applied on a current collector made of aluminum foil having athickness of 20 μm using a coating machine to conduct vacuum dry at 80°C. for 12 hours.

Thereafter, it was punched in a φ 14 mm disk shape and was pressed usinga hand press at 16 MPa for 2 minutes to produce a positive electrode.

Subsequently, a coin type secondary cell was assembled using thepositive electrode. As a negative electrode, lithium foil punched in a φ15 mm disk shape was used. As an electrolytic solution, a solutionobtained by dissolving LiPF₆ (in the case of lithium ion secondary cell)or NaPF₆ (in the case of sodium ion secondary cell) in a mixed solventobtained by mixing ethylene carbonate and ethyl methyl carbonate in avolume ratio of 1:1 so that the concentration of LiPF₆ or NaPF might be1 mol/L was used. As a separator, a publicly known separator such as aporous polymer film such as polypropylene was used. These cell partswere incorporated and accommodated under an atmosphere in which the dewpoint thereof is −50° C. or less by an ordinary method to produce thecoin type secondary cell (CR-2032).

Charge and discharge tests were conducted using the produced secondarycells. In the case of the lithium ion cell, the discharge capacity at 1CA was determined setting the charge conditions to constant current andconstant voltage charge at a current of 1 CA (330 mA/g) and a voltage of4.5 V and setting the discharge conditions to constant current dischargeat 1 CA (330 mA/g) and a final voltage of 1.5 V. In the case of thesodium ion cell, the discharge capacity at 1 CA was determined settingthe charge conditions to constant current and constant voltage charge ata current of 1 CA (154 mA/g) and a voltage of 4.5 V and setting thedischarge conditions to constant current discharge at 1 CA (154 mA/g)and a final voltage of 2.0 V. Further, repeated tests of 50 cycles wereconducted under the similar charge-discharge conditions to determinecapacity retention rates (%) in accordance with the following formula(2). It is to be noted that all the charge and discharge tests wereconducted at 30° C.

Capacity retention rate (%)=(discharge capacity after 50cycles)/(discharge capacity after 1 cycle)×100  (2)

The results are shown in Table 2.

TABLE 2 Initial discharge Capacity capacity retention at 1 C. (mAh/g)rate (%) Example 1-1 155 92.5 Example 1-2 156 93.2 Example 1-3 155 95.0Example 1-4 145 91.0 Example 1-5 140 92.0 Comparative Example 1-1 15286.0 Example 2-1 201 28.0 Example 2-2 211 32.0 Example 2-3 199 27.0Example 2-4 184 21.0 Comparative Example 2-1 200 19.0 Example 3-1 11691.0 Example 3-2 123 93.0 Example 3-3 115 91.0 Example 3-4 104 87.0Comparative Example 3-1 112 85.0

From the results, it found that the positive electrode active substancesof Examples can reduce the amount of the adsorbed water more surely andcan exhibit more excellent performance in the obtained cells, ascompared with the positive electrode active substances of ComparativeExamples.

1: A positive electrode active substance, wherein a water-insolubleelectrically conductive carbon material and carbon obtained bycarbonizing a water-soluble carbon material are supported on a compoundcomprising iron or manganese, wherein the compound is represented byformula (A), (B), or (C):LiFe_(a)Mn_(b)M¹ _(c)PO₄  (A) wherein M¹ represents Mg, Ca, Sr, Y, Zr,Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and a, b, and c each represent anumber satisfying 0≦a≦1, 0≦b≦1, 0≦c≦0.2, 2a+2b+(valence of M¹)×c=2, anda+b≠0;Li₂Fe_(d)Mn_(e)M² _(f)SiO₄  (B) wherein M² represents Ni, Co, Al, Zn, V,or Zr, and d, e, and f each represent a number satisfying 0≦d≦1, 0≦e≦1,0≦f<1, 2d+2e+(valence of M²)×f=2, and d+e≠0; andNaFe_(g)Mn_(h)Q_(i)PO₄  (C) wherein Q represents Mg, Ca, Co, Sr, Y, Zr,Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and g, h, and i each represent anumber satisfying 0≦g≦1, 0≦h≦1, 0≦i<1, 2g+2h+(valence of Q)×i=2, andg+h≠0. 2: The positive electrode active substance according to claim 1,wherein the carbon obtained by carbonizing the water-soluble carbonmaterial is supported on a composite comprising the compound and thewater-insoluble electrically conductive carbon material. 3: The positiveelectrode active substance according to claim 1, wherein an amount ofthe water-insoluble electrically conductive carbon material and thewater-soluble carbon material expressed in terms of carbon atoms is 1.0to 20.0 mass % in total. 4: The positive electrode active substanceaccording to claim 1, wherein the water-soluble carbon material is atleast one selected from the group consisting of a saccharide, a polyol,a polyether, and an organic acid. 5: The positive electrode activesubstance according to claim 1, wherein the water-insoluble electricallyconductive carbon material is graphite. 6: The positive electrode activesubstance according to claim 2, wherein in the composite, thewater-insoluble electrically conductive carbon material is supported onthe compound by subjecting the water-insoluble electrically conductivecarbon material and the compound obtained through hydrothermal reactionto dry mixing. 7: The positive electrode active substance according toclaim 6, wherein the dry mixing is mixing wherein the compound and thewater-insoluble electrically conductive carbon material are subjected topreliminary mixing and subsequently mixed while compressive force andshear force are applied. 8: The positive electrode active substanceaccording to claim 2, wherein the water-soluble carbon material issupported on the compound as carbon obtained through carbonization bysubjecting the water-soluble carbon material and the compositecomprising the compound to wet mixing and then pyrolyzing. 9: A methodfor producing a positive electrode active substance, the positiveelectrode active substance being one wherein a water-insolubleelectrically conductive carbon material and carbon obtained bycarbonizing a water-soluble carbon material are supported on a compoundcomprising iron or manganese, wherein the compound is represented byformula (A), (B), or (C):LiFe_(a)Mn_(b)M¹ _(c)PO₄  (A) wherein M¹ represents Mg, Ca, Sr, Y, Zr,Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and a, b, and c each represent anumber satisfying 0≦a≦1, 0≦b≦1, 0≦c≦0.2, 2a+2b+(valence of M¹)×c=2, anda+b≠0;Li₂Fe_(d)Mn_(e)M² _(f)SiO₄  (B) wherein M² represents Ni, Co, Al, Zn, V,or Zr, and d, e, and f each represent a number satisfying 0≦d≦1, 0≦e≦1,0≦f<1, 2d+2e+(valence of M²)×f=2, and d+e≠0; andNaFe_(g)Mn_(h)Q_(i)PO₄  (C) wherein Q represents Mg, Ca, Co, Sr, Y, Zr,Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and g, h, and i each represent anumber satisfying 0≦g≦1, 0≦h≦1, 0≦i<1, 2g+2h+(valence of Q)×i=2, andg+h≠0, and the method comprising: (I) subjecting a slurry comprising: alithium compound or a sodium compound; a phosphoric acid compound or asilicic acid compound; and a metal salt comprising at least an ironcompound or a manganese compound to hydrothermal reaction, therebyobtaining a compound X; (II) adding a water-insoluble electricallyconductive carbon material to the obtained compound X and conducting drymixing, thereby obtaining a composite Y; and (III) adding awater-soluble carbon material to the obtained composite Y before wetmixing and then conducting pyrolysis. 10: The method for producing apositive electrode active substance according to claim 9, wherein thewater-soluble carbon material is at least one selected from the groupconsisting of a saccharide, a polyol, a polyether, and an organic acid.11: The method for producing a positive electrode active substanceaccording to claim 9, wherein the water-insoluble electricallyconductive carbon material is graphite. 12: The method for producing apositive electrode active substance according to claim 9, wherein in(II), the water-soluble carbon material is added together with thewater-insoluble electrically conductive carbon material. 13: The methodfor producing a positive electrode active substance according to claim9, wherein an amount of the water-soluble carbon material added in (III)is 1.0 to 55.0 mass parts based on 100 mass parts of the composite Y.14: The method for producing a positive electrode active substanceaccording to claim 9, wherein the dry mixing in (II) is mixing whereinthe compound and the water-insoluble electrically conductive carbonmaterial are subjected to preliminary mixing and subsequently mixedwhile compressive force and shear force are applied. 15: The positiveelectrode active substance according to claim 1, wherein the compound isrepresented by the formula (A). 16: The positive electrode activesubstance according to claim 1, wherein the compound is represented bythe formula (B). 17: The positive electrode active substance accordingto claim 1, wherein the compound is represented by the formula (C).